INHIBITION OF SQUALENE MONOOXYGENASE BY TELLURIUM AND SELENIUM COMPOUNDS

Tellurium (Te) is a metalloid element located directly below selenium (Se), and sulfur, on the Periodic Table of the elements.  Tellurium is a relatively rare element, most often found in combination with ores such as copper and silver.  Telluride, Colorado is named after this element.  Tellurium is currently used in semiconductor fabrication and metallurgy; there are no known biological roles for tellurium.  Although rarely encountered except in industrial settings, tellurium is relatively toxic, with acute exposure causing a variety of gastrointestinal and neurological symptoms, and death in severe exposures.  Most characteristic of tellurium intoxication is a garlicky odor on the breath, due to the volatilization of dimethyltelluride formed in the liver.¹  

Although selenium (Se) shares many chemical characteristics with tellurium, its biology is very different.  Selenium is a required trace element and is a key component of several enzymes involved in antioxidant defense (glutathione peroxidases, thioredoxin reductase) and metabolism (iodothyronine deiodinases).²  In animals selenium is incorporated into proteins as the amino acid selenocysteine, where selenium replaces the sulfur normally found in cysteine.³  This enhances the redox properties of this amino acid.  Although an essential trace element, selenium is toxic in doses only slightly greater than those recommended for good health.  Selenium toxicity at one time was common in range cattle in the upper Great Plains of the U.S. due to the high content of selenium in the soil, and is still occasionally encountered in livestock fed improperly prepared feed supplements.  Selenium toxicity has also been documented in man.⁴  Toxicity is characterized by loss of hair and hooves (or nails in man), gastrointestinal disturbances, ataxia, and neurological symptoms including demyelination and parasthesias.⁵  Excess selenium consumption can also produce a garlicky odor on the breath.  Selenium exists in several oxidation states: selenite (SeO₃⁼) is considered to be the toxic form of this element, whereas selenate (SeO₄⁼) is relatively unreactive in biological systems, although some plants can take up selenate and incorporate it into selenocysteine and selenomethionine.⁵

Feeding tellurium to weanling rats causes a transient, hind-limb paralysis due to demyelination of the sciatic nerve.⁶  This observation in 1970 lead to series of studies from Pierre Morell's laboratory at the University of North Carolina that traced the cause of this demyelination to the inhibition of cholesterol synthesis, and the accumulation of squalene, in Schwann cells.⁷  Cholesterol is an abundant component of the myelin sheath that surrounds nerves and facilitates the rapid neuronal transmission of signals.  When the myelin sheath breaks down neurotransmission is severely impeded.  The enzyme that is inhibited by tellurium is squalene monooxygenase, the second enzyme in the downstream pathway for cholesterol biosynthesis.  Morell's group showed that this enzyme is inhibited by tellurium feeding, and by tellurite in vitro; more recent studies by Jeffrey Goodrum have indicated that the dimethyl metabolite of tellurium is likely to be the inhibitory species formed in vivo upon tellurium ingestion.⁸  My laboratory has investigated the mechanism of this inhibition, as described below.

Tellurite (TeO₃⁼) and selenite inhibit purified human squalene monooxygenase with IC₅₀ values (50% inhibition) of 17 and 37 µM, respectively, while arsenite (AsO₃⁼) and selenate are not inhibitory.⁹    These studies confirm earlier studies with subcellular preparations (microsomes), and demonstrate that the human enzyme is also susceptible to inhibition by these chemicals.  Moreover, selenite was found to be a reasonable inhibitor of this enzyme.

The studies of Goodrum⁸ indicated that tellurite may not be the actual inhibitor in vivo after tellurium ingestion, but rather that a dimethylated metabolite of tellurium is responsible for the inhibition.  Inorganic tellurium and selenium are methylated in the liver by several methyltransferases prior to excretion.  The principal metabolite of either element is the trimethyl ion, which is excreted from the body in the urine.  The dimethyl metabolite, which is presumably an intermediate in the methylation pathway, accumulates when intake of either element is excessive.  These dimethyl intermediates are volatile and appear on the breath, giving rise to a garlicky odor.  

We examined the ability of dimethyltelluride (DMT) and dimethylselenide (DMS) to inhibit human squalene monooxygenase, as shown to the right.  Dimethyltelluride is a potent inhibitor of this enzyme, with an IC₅₀ value of approximately 100 nM, 170-fold more potent than tellurite.¹⁰  In contrast, dimethylselenide is a relatively weak inhibitor, with an IC₅₀ value of 680 µM, 18-fold less potent than selenite.¹¹  These studies reveal that metabolism of tellurium and selenium leads to very different outcomes: methylation of selenium is a detoxification reaction, whereas it increases the toxicity of tellurium.  Further studies described below investigate the mechanism by which these compounds inhibit squalene monooxygenase.

The kinetics of inhibition by tellurium and selenium revealed that these compounds are slow, time-dependent inhibitors, suggestive of irreversible binding to the enzyme.  This was confirmed by enzyme dilution experiments, in which reducing the inhibitor concentration 10-12-fold by dilution could not reverse the inhibition, as shown to the right.  Irreversible inhibition often suggests covalent binding of the inhibitor to the enzyme.  As cysteine sulfhydryls are often targets of this binding,  this reaction can often be blocked by the addition of thiols to the incubation.

As shown to the right, the addition of glutathione (GSH), a monothiol, or dimercaptopropanol (DMP), a dithiol, to the incubations was able to block the inhibition by the tellurium and selenium compounds, presumably by binding to the inhibitors before they could react with the enzyme cysteine sulfhydryls.  In some cases this enzyme inactivation also could be reversed by the addition of these thiols, as shown below:

Thiols, and especially the dithiol DMP, can reactivate squalene monooxygenase that has been inhibited by tellurite, as shown to the right.  This suggests that DMP can break the cysteine-tellurite bond and release tellurite from the enzyme.  However, enzyme inhibited by DMT cannot be reactivated, suggesting that this inhibitor-enzyme bond cannot be readily broken by added thiols.  Inhibition by selenite presents a third scenario: GSH can reactivate the enzyme, presumably releasing selenite from the enzyme, whereas DMP significantly enhances the inhibition by this compound.  DMP may convert selenite into a more reactive inhibitor; selenide is one such possibility. 

Although chemically very similar, tellurium and selenium compounds react very differently with the cysteine sulfhydryl groups on squalene monooxygenase.  Dimethyltelluride is a potent inhibitor of this enzyme, whereas dimethylselenide is largely unreactive.  Enzyme inhibited by tellurite is more readily reactivated by dithiols than monothiols, suggesting that tellurite binds to vicinal cysteines; inhibition of squalene monooxygenase by phenylarsine oxide, a known substrate for vicinal cysteines, is also readily reversed only by DMP.¹⁰  In marked contrast, selenite inhibition cannot be reversed by dithiols, and in fact is increased in the presence of these reductants.¹¹  The dithiol-reduced selenite product (selenide?) may resemble DMT in that inhibition by these compounds is unaffected by thiol reagents once the inhibitor-enzyme bond is formed (enzyme inhibited by DMT cannot be reactivated by either monothiols or dithiols).  Why these similar tellurium and selenium compounds have such different behavior toward squalene monooxygenase is the subject of current investigations in my laboratory.

These results may have significant implications for the toxicity of tellurium and selenium.  Although tellurium is rarely encountered in normal settings, selenium is often taken as a vitamin supplement.  Although selenium supplementation has many beneficial effects, including antioxidant defenses and anticancer effects,² excessive intake could lead to toxic effects through the binding of selenite and its biological metabolites (selenide and methylselenol) to sulfhydryl-sensitive enzymes, including squalene monooxygenase in neural tissues.

For more information on the inhibition of human squalene monooxygenase by tellurium and selenium, access the abstracts of our publications on this work. 

1. Laden BP, Tang Y, Porter TD.  Cloning, heterologous expression, and enzymological characterization of human squalene monooxygenase.  Arch Biochem Biophys 2000 Feb 15;374(2):381-8 [abstract]
    
2. Laden BP, Porter TD.  Inhibition of human squalene monooxygenase by tellurium compounds: evidence of interaction with vicinal sulfhydryls.  J Lipid Res 2001 Feb;42(2):235-40 [abstract]
    
3. Gupta N, Porter TD.  Inhibition of human squalene monooxygenase by selenium compounds.  J Biochem Mol Toxicol 2002;16(1):18-23 [abstract]

1. Reuhl KR, Polunas MA.  Tellurium,  in Experimental and Clinical Neurotoxicology, 2nd Edition, PS Spencer, HH Schaumburg, AC Ludolph, Eds., Oxford University Press, 2000, pp. 1140-43.  Larner AJ.  Biological effects of tellurium: a review.  Trace Elem. Electrolytes 12, 26-31, 1995.
2. Kohrl J, Brigelius-Flohe R, Bock A, Gartner R, Meyer O, Flohe L.  Selenium in biology: facts and medical perspectives.  Biol Chem 2000 Sep-Oct;381(9-10):849-64 [abstract].  Holben DH, Smith AM.  The diverse role of selenium within selenoproteins: a review.  J Am Diet Assoc 1999 Jul;99(7):836-43 [abstract].  
3. Stadtman TC.  Selenocysteine.  Annu Rev Biochem 1996;65:83-100 [abstract]
4. Yang GQ, Wang SZ, Zhou RH, Sun SZ.  Endemic selenium intoxication of humans in China.  Am J Clin Nutr 1983 May;37(5):872-81 [abstract]
5. Raisbeck MF.  Selenosis.  Vet Clin North Am Food Anim Pract 2000 Nov;16(3):465-80 [abstract].  Barceloux DG.  Selenium.  J Toxicol Clin Toxicol 1999;37(2):145-72 [abstract].  Wilber CG.  Toxicology of selenium: a review.  Clin Toxicol 1980 Sep;17(2):171-230 [abstract].
6. Lampert P, Garro F, Pentschew A.  Tellurium neuropathy.  Acta Neuropathol (Berl) 1970;15(4):308-17.  Lampert PW, Garrett RS.  Mechanism of demyelination in tellurium neuropathy. Electron microscopic observations.  Lab Invest 1971 Nov;25(5):380-8
7. Harry GJ, Goodrum JF, Bouldin TW, Wagner-Recio M, Toews AD, Morell P.  Tellurium-induced neuropathy: metabolic alterations associated with demyelination and remyelination in rat sciatic nerve.  J Neurochem 1989 Mar;52(3):938-45 [abstract].  Wagner-Recio M, Toews AD, Morell P.  Tellurium blocks cholesterol synthesis by inhibiting squalene metabolism: preferential vulnerability to this metabolic block leads to peripheral nervous system demyelination.  J Neurochem 1991 Dec;57(6):1891-901 [abstract].  Wagner M, Toews AD, Morell P.  Tellurite specifically affects squalene epoxidase: investigations examining the mechanism of tellurium-induced neuropathy.  J Neurochem 1995 May;64(5):2169-76 [abstract].  
8. Goodrum JF.  Role of organotellurium species in tellurium neuropathy.  Neurochem Res 1998 Oct;23(10):1313-9 [abstract]
9. Laden BP, Tang Y, Porter TD.  Cloning, heterologous expression, and enzymological characterization of human squalene monooxygenase.  Arch Biochem Biophys 2000 Feb 15;374(2):381-8 [abstract]
10. Laden BP, Porter TD.  Inhibition of human squalene monooxygenase by tellurium compounds: evidence of interaction with vicinal sulfhydryls.  J Lipid Res 2001 Feb;42(2):235-40 [abstract]
11. Gupta N, Porter TD.  Inhibition of human squalene monooxygenase by selenium compounds.  J Biochem Mol Toxicol 2002;16(1):18-23 [abstract]

For other information on squalene monooxygenase:

• Biology of squalene monooxygenase

  • Discovery and Characterization

  • Regulation of Expression

  • Therapeutic and Natural Inhibitors
    

• The squalene monooxygenase family and related enzymes

  • Squalene Monooxygenase Sequences

  • Alignment of Squalene Monooxygenases

  • Other Flavoproteins Related to Squalene Monooxygenase

  • Comparison of Squalene Monooxygenase to p-Hydroxybenzoate Hydroxylase

  • The 3-Dimensional Structure of p-Hydroxybenzoate Hydroxylase
    

• Enzymology of squalene monooxygenase

  • Mechanism

  • Interaction with Cytochrome P450 Reductase

  • Interaction with Squalene and FAD
    

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